Repeaters are used to enable telephoning with a mobile communications terminal in areas that are shadowed from radio waves, such as, for example, in a tunnel.
Such a repeater includes in its usual design, a master unit, which is in wireless or wire-bound communication connection with a base station of a mobile network. The repeater also includes several “remote units”, for example, distributed over the length of the tunnel, for establishing the contact with the mobile communications terminal. Signal transmission between the master unit and the remote units is often effected as an amplitude-modulated optical analog signal via one or several optical waveguides, for example optical fiber or glass cables.
The master unit transfers the signal supplied to it by the base station to all remote units in the “downlink” direction. Each remote unit, on the other hand, transfers the signal supplied to it by a mobile communications terminal to the master unit in the “uplink” direction. An undisturbed signal transmission requires a separation of the individual signals, namely, on the one hand, of the downlink-signals from the uplink-signals, and, on the other hand, of the individual uplink-signals of the various remote units from each other.
This is achieved, for example, by transmitting each signal through a separate optical waveguide to a separate optical receiver, which is, however, relatively cost-intensive.
Alternatively, it is possible, in particular in the uplink direction, to use one common optical waveguide for the signal transmission of several remote units. To achieve a separation of the optical signals, each of the remote units conventionally includes a transmitter such as a laser, with each of the lasers having a nominal wavelength different from that of the other lasers and emitting light of a nominal wavelength. That is, a spectral separation of the uplink signals is effected, as each remote unit transmits on a different light wavelength. It is, however, a disadvantage in this type of data transmission that for each remote unit, an individual laser type has to be provided. This complicates in particular the maintenance of such a repeater.